In a conventional image forming apparatus of the electrophotographic type, in general, a surface of a drum-like photosensitive member as an image bearing member is electrically charged uniformly by a charger and then the charged photosensitive member is exposed to light depending on image information by an exposure device to form an electrostatic latent image on the photosensitive member. The electrostatic latent image formed on the photosensitive member is visualized as a toner image by a toner contained in a developer by using the developing device.
As such a developing device, there is a developing device using, as the developer, a two-component developer including non-magnetic toner particles (toner) and magnetic carrier particles (carrier). Particularly, in a color image forming apparatus, the toner may contain no magnetic material and therefore the two-component developer has been widely used for the reason such that color (tilt) is good or the like.
In such a developing device, in general, a regulating blade as a layer thickness regulating member is provided so as to be opposed to an outer peripheral surface of a developing sleeve through a predetermined gap in many cases. The developer carried on the developing sleeve is subjected to regulation of an amount thereof to be fed to a developing region in a process in which the developer passes through a gap between a developing sleeve 8 and a regulating blade 9 when the developer is fed to the developing region, so that the developer is adjusted so as to be fed (supplied) in a stable amount.
However, in the developing device in which the layer thickness regulation of the developer carried on the developing sleeve surface is effected by the regulating blade, the following problem can arise. FIG. 5 is a schematic sectional view showing a state of the two-component developer at a position upstream of the position of the regulating blade in the case where the conventionally known two-component developer is used. By a magnet incorporated in the developing sleeve, the developer is carried and fed to develop the electrostatic (latent) image. In such a developing device, the developer portion is divided into a portion where a flow of the developer is stopped by the regulating blade and a portion where the developer follows rotation of the developing sleeve to be fed at substantially the same speed as a rotational speed of the developing sleeve, so that a shear surface (plane) is generated at a boundary portion. A developer A located on the shear surface is pressed against the regulating blade by a circumferential force with the rotation of the developing sleeve, so that the developer is in a packed state and then is continuously stagnated in some cases. In the case where the developer on the shear surface is stagnated for a long term, at the boundary surface, a mobile developer layer and an immobile developer layer rub with each other. As a result, the toner is liberated from the carrier by the rubbing in the case of the two-component developer and then the liberated toner particles are liable to be adhered to each other by frictional heat due to the rubbing, thus forming the toner layer. The thus formed toner layer grows by continuous rotation of the developing sleeve 8, so that the gap between the regulating blade 9 and the developing sleeve 8 is obstructed, and thus the amount of the developer passing through the gap is lowered (hereinafter this phenomenon is referred to as improper coating). As a result, the amount of the developer conveyed to the developing region fluctuates, so that problems such as a density lowering and longitudinal density non-uniformity were generated.
In Japanese Laid-Open Patent Application (JP-A) Hei 5-035067, in order to prevent the formation of the immobile layer of the developer, provision of a cylindrical toner feeding member which steadily rotates always with a certain gap with the developing sleeve in an immediately upstream side of the regulating blade is proposed.
However, in JP-A Hei 5-035067, the formation of the immobile layer of the developer can be prevented but a bearing for supporting the toner feeding member and a driving means for driving the toner feeding member are required, so that it is inevitable that a constitution is complicated and a cost therefor is increased. In addition, the toner feeding member is driven in an opposite direction at a position where it opposes the developing sleeve and therefore strong stress is imposed on the developer, so that there is a possibility that the developer is deteriorated early.